The present invention relates broadly to light transmission media and to associated lighting devices and improvements in lighting devices incorporating light emitting diodes as light sources, particularly the use of an ethylene/butene-1 copolymer or a propylene homopolymer to serve as a modifier to deliver a more uniform light output across the lighting device which allows the light to be diffused and magnified.
Light emitting diodes (LEDs) have been used for many purposes for many years. Generally they have been used as indicator lights and in arrays to form information displays. The light emitting diodes have the advantages of operating at a low voltage, approximately three volts, and energy efficiency, generating approximately 25 lumens of light per watt. The disadvantage of this type of lighting is that the light is unidirectional, that is, the light is directed only in the direction of the axis of the light emitting diode. The light does not diffuse to the sides. Typical LEDs provide light less than 40xc2x0 from the axis of the diode. If the LED is being used in a warning light to alert the public to a danger, a person approaching the warning light from the side will not have their attention drawn to the peril.
The light from an LED is also concentrated, appearing to emanate from a small point. Currently many warning and signal lights constructed with LEDs do not meet specifications set by the Institute of Transportation Engineers. This is because the LEDs emit a highly concentrated intense unidirectional light causing a xe2x80x9chotxe2x80x9d spot in the center of the signal face and the testing standard measures the light intensity over an area +/xe2x88x9210xc2x0 from the optical center. In some cases an array of LEDs is used to provide a larger signal area, but such arrays are wasteful as they require more LEDs, increasing production costs and requiring more energy to operate.
Presently, incandescent light sources are widely used in existing lighting devices. Examples of these products are exit lights, flashing yellow barrier lights, lighted exit signs required in buildings, Christmas lights, hand held signal lights such as those used by traffic police, and instrument panel display lights. Because these devices use incandescent light bulbs, they are inefficient. They are also susceptible to vibration, which may cause a failure or a shortening of the bulbs useful life. The mean time between failure for a typical incandescent device is less than 5000 hours. A typical LED has a mean time between failure of 100,000 hours.
The following table summarizes the comparison of current designs using various light sources:
It is an object of this invention to provide a reduced power light source for warning devices, signs, and other illuminated articles. It is also an object of this invention to provide an omnidirectional LED light source and a device that gives a more uniform light output across the face of the device compared to the unidirectional concentrated light of traditional LED light devices.
This invention need not be restricted to LEDs as the light source. It can also utilize any light source, both visible and invisible such as infra-red, such as an incandescent, flourescent, or neon bulb, or lasers.
The device of this invention is an omni-directional light assembly that emits light from the side as well as in the direction of the axis of the light source. The light is dispersed and magnified by a light transmitting material herein disclosed so that persons approaching such a device from the side would clearly see the emitted light. More particularly, the invention is directed towards an omni-directional light assembly utilizing a non omni-directional light source, such as an LED.
The invention, when utilizing LEDs or other semiconductors, further provides the benefit of reduced power consumption over incandescent bulbs. The invention may further gain energy efficiency by utilizing solid state circuitry to pulse at a rate sufficiently high so as to appear to be steadily on, or in the alternative, pulse with and desired rate to create a flashing light. Such a circuit is disclosed in co-owned U.S. patent application Ser. No. 08/802,526 , now U.S. Pat. No. 5,879,076.
The invention has application in areas such as traffic safety barriers, logos or emblems, hand held light signals, exit or other warning and signal lights, safety articles as disclosed in U.S. patent application Ser. No. 08/802,526, now U.S. Pat. No. 5,879,076, as well as instrumentations lighting.
The apparatus of the present invention comprises the LED itself with a cover made of a plastic material such as ethylene or propylene. The cover can be clear or it can contain color. It may also be translucent or opaque, depending upon the formulation and method of production. Since the normal LED is small, a cover having a diameter in the range of about 3 inches would be satisfactory to cover the emitted light from the LED and provide the omni-directional condition for a warning light. The specific dimension of any article will vary with the application. It should be obvious that the cover can be made to any shape and size. As the size increases one may desire to use multiple LEDs.
The spatial relationship of the cover and the light source may vary. The cover may be spaced from the light source, as is the common design of typical incandescent lights and associated lenses. Alternatively, the light source can be placed against or in contact with the cover. An extension of this design is embedding the light source in the cover material. The LED light source lends itself well to this arrangement since it is small and gives off negligible heat.
The cover is made from a plastic material, preferably from a polyolefin and more preferably a polyolefin in either a rigid or semi-rigid state. The polyolefin is mixed with a hydrocarbon material to create a matte look. Broad ranges of hydrocarbons can be used for this purpose. A clarifying agent may also be added. An example of such an agent is Millad 3988 made by Milliken Chemical Company which is added at about 0.25% by weight of the polyolefin material. The cover is generally molded or blow molded from the polyolefin material in a conventional manner using heat or it may be extruded.
A particular polyolefin material which has been found useful in the invention is a propylene homopolymer designed for injection molding applications. This particular propylene homopolymer has a melt flow rate of 32 grams per 10 minutes, a density of 0.90 grams per cm3, a melting point of 147xc2x0 C., a tensile strength of 5.0 kpsi, an elongation of 16% and a flexural modulus of 200 kpsi. Such a compound is manufactured and sold by Exxon Chemical under the trademark ACHIEVE 1625.
A particular ethylene/butene-1 copolymer material which has been found useful in the invention is a copolymer designed for injection molding applications. Such a compound is manufactured and sold by Exxon Chemical under the trademark EXACT 4049.
The EXACT 4049 material is flexible and milky in color. The flexablility allows the lighting device made from the material to be easily bent or shaped into desired configurations. The ACHIEVE material is rigid and also milky in color. The ACHIEVE material is appropirate for applications where a rigid cover is desired. The two materials are compatible and may be mixed in any combination to result in a cover of desired rigidity or flexablility.
The ACHIEVE material is normally milky in color and transparent. The addition of a material known as Millad Concentrate 8C41-10, manufactured by Milliken chemical of Inman S.C. and belonging to the chemical family of sorbitol acetal in polyolefin blend, is known to act as a clarifying agent if added in a ratio of 25% by weight of the polyolefin material. One skilled in the art will readily recognize that other clarifying agents may be used.
The EXACT 4049 material may also be rendered clear by rapidly chilling the product immediately upon extrusion or molding. When the material is extruded or molded, it is typically at a temperature of 140xc2x0 to 200xc2x0 F. If allowed to cool at room temperature, the material will have a milky but translucent appearance. If the material is rapidly cooled however, the resulting product will be clear. The preferred way to cool the material is to bathe the product in chilled water just after it is extruded or molded and is still above 140xc2x0 F. While there are many ways to accomplish the cooling, the preferred way is to spray chilled water upon the product. The water should be at a temperature just above freezing for the best results.
The cover described in this application is useable with numerous different kinds of LEDs. When the LED itself is not circular in its upper dimension the cover of this invention can be molded in different forms so that it is useful with any kind of an LED.
The cover may be shaped or molded into any number of desired shapes. Also, the LED may be placed anywhere within or at the surface of the cover. More than one LED may be used as a light source if the face of the device is large or if higher intensity light is desired.
The lights source and cover combination has many uses. For instance, the cover may be used with an incandescent bulb and suitable power source to from a signal or warning light, such as those commonly seen in highway construction zones. While the cover of the present invention would perform well with an incandescent light source, it would be preferable to utilize a light source of lower power consumption and longer life, such as an LED. LEDs have not been used in the past for barrier warning lights because of the unidirectional and concentrated nature of the light emitted. The cover modifies the concentrated light of the LED and spreads it out across the surface of the cover, resulting in an omnidirectional uniform lighting effect. Thus the cover allows use of an LED or an array of LEDs where incandescent lights were the preferred source. Suitable electronic circuitry may be electrically connected to the light source and power source to cause the light to flash at a desired time interval. Such circuits are well known to those skilled in the art.
The cover and LEDs and can be combined with a standard incandescent bulb base to create a replacement bulb for the incandescent bulb. Such a replacement bulb utilizing LEDs as a light source instead of a filament can provide a operational cost advantage.